A naval clash in the Gulf has reignited fears over the security of the world's most important shipping lanes and disputed oilfields.

By Richard Spencer in Dubai

6:00AM GMT 26 Mar 2010

The United Arab Emirates navy is thought to have opened fire on a small patrol vessel from Saudi Arabia after a dispute over water boundaries.

According to one report, two Saudi sailors were injured in the alleged bombardment.

The Saudi vessel was forced to surrender, and its sailors were delivered into custody in Abu Dhabi for several days, before being released and handed over to the Saudi embassy earlier this week.

The incident has shocked diplomats who hope the countries, both key American allies, will help implement the West's strategy to constrain Iran's nuclear and military ambitions.

The clash happened in disputed waters between the coasts of Saudi Arabia, Abu Dhabi and the peninsula on which the gas-rich state of Qatar sits.

The seabed is rich with oil deposits, while the Dolphin pipeline project to carry natural gas direct from Qatar to Abu Dhabi has provoked irritation in the Saudi authorities. Nevertheless, direct conflict between the two countries' armed forces is highly unusual.

The Gulf is one of the most heavily armed regions in the world. The Saudi government has been building up its army and air force for years in response to what it sees as a regional threat from Iran.

The UAE was slower to join the arms race, despite a long-running row with Iran over three Gulf islands previously under Abu Dhabi control which were seized by the late Shah in 1971 on the night the Emirates celebrated their independence.

But now the UAE, despite its small size, is the fourth largest purchaser of weaponry on the international market in the world.

Western governments are exasperated that the two countries are unable to co-operate because of a series of long-running border disputes, largely influenced by oil reserves.

Saudi Arabia is the world's largest oil producer, while Abu Dhabi, though ranking only number four in OPEC, is by some counts the richest city per head of population in the world.

"It looks as though attempts were made to keep this quiet, which is predictable given the important relationship between the two countries and the strategic relationship with Iran," a Gulf-based diplomat said. "But it does remind us of the simmering rows that there are in this part of the Gulf."

The Gulf is the shipping route for 40 per cent of the world's oil trade. The lack of agreed naval boundaries leads to repeated arrests of civilian vessels, including a British yacht by the Iranian navy last November, but more serious is the threat of Iranian retaliation for any attack by Israel or American forces on its nuclear installations.

The Iranian government has threatened to mine the Straits of Hormuz at the tip of the Gulf, or target the western navies moored in Gulf Arab ports.

"This is getting serious," a local defence analyst said. "The Dolphin pipeline is a critical interstate energy project to bring gas from Qatar to the UAE, so a fight (with Saudi Arabia) is affecting the relations between these three countries at a time when they should be co-operating."

A spokesman for the UAE ministry of defence said he was unable to give details of the incident.